At 6:23 a.m. on June 2, Beijing time, the Chang'e-6 probe landed/ascended in the pre-selected area of the South Pole of the Moon-Aitken Basin, and successfully arrived at the "Moon Palace" after a 30-day long journey, opening a new chapter in sampling on the back of the moon.
Flying away from the earth, traveling in space, and landing on the moon steadily, this seemingly easy scene is actually a difficult one.
If the landing of a probe on the moon for scientific exploration is regarded as a feat of climbing Mount Everest, then flying from the earth to orbiting the moon is roughly equivalent to reaching the assault camp from the forward camp, and breaking away from the orbit around the moon, gradually slowing down, and choosing a safe location to carry out a soft landing on the surface of the moon, which is equivalent to the process of finally hitting the summit - although the distance is not long, but the difficulty and risk are the most dangerous, and the effectiveness and reliability of power technology are also the most tested.
In order to successfully land on the Moon, the landing/ascent assembly of the Chang'e-6 probe needs to start a dynamic descent from 15 kilometers above the lunar surface, and the speed of the combination gradually decreases from 1.6 kilometers per second relative to the Moon to zero. Just imagine how thrilling it would be for a 4-ton "behemoth" to land from a height of 15 kilometers!
The Sixth Academy of Aerospace Science and Technology Group has developed a propulsion subsystem for the Chang'e-6 lander, including a 7500N variable thrust engine, a 150N engine, and more than 10 other products such as storage tanks and gas cylinders. When Chang'e-6 is assisted in the power descent, the speed is reduced from about 1.6 kilometers per second to 0 (i.e., hovering), and the altitude is reduced from 15 kilometers to less than 3 kilometers, so as to achieve deceleration. Only when the engine is "pushed steadily" can the speed of Chang'e-6 "become accurate".
Soft landing, hard kung fu
Whether the brakes will be applied correctly is one of the important indicators to judge the quality of a driver's driving skills. Similarly, in order to reduce the impact on the probe during the landing and deliver the "passengers" smoothly, the "brakes" must be just right.
In the process of landing on the moon, the landing/ascent combination of the Chang'e-6 probe should be like an experienced "old driver", while flying forward and descending, quickly adjust the posture and adjust the speed, adjust to the vertical descent at an altitude of 3 kilometers from the lunar surface, the 7500N engine began to work with variable thrust, and then continued to descend, and when it was found that 100 meters below was the lunar surface, the obstacle was avoided through the transverse 150N engine, in order to ensure that each "foot" of Chang'e-6 did not step into the pit when it landed on the moon. The combination will take a short "rest" at a position 3 meters above the lunar surface, and after selecting the landing site by hovering and obstacle avoidance, the 7500N engine will be shut down, and Chang'e-6 will slowly fall under the action of lunar gravity. This steady fall seems to be light and relaxed, but it contains a lot of wisdom and accumulation of the engine products of the Sixth Academy.
As the key single unit of the probe, the 7500N variable thrust engine is the main driving force for the power descent of the Chang'e-6 probe. It is not simple, it is an engine that can not only operate at fixed thrust, but also throttle and reduce thrust according to demand, and it is a necessary device for the landing of extraterrestrial objects. The 7500N variable thrust engine is the first variable thrust engine in mainland China developed by the Sixth Academy, which can realize a wide range of continuous changes in thrust from 1500N to 7500N, and has the characteristics of high performance, strong adaptability, stable combustion and simple structure. The thrust can be changed accurately, quickly and steplessly according to the control instructions of the probe, so as to realize the mid-course correction, near-moon braking and soft landing of the probe. This type of engine has helped Chang'e-3, 4 and 5 successfully land on the moon three times in a row, with a success rate of 100%.
The "blind landing" on the back of the moon is frightening step by step
The more actions there are, the greater the risk. There is no doubt that Chang'e-6 will be the most complex lunar exploration journey, but the variables are the challenges. As the engine's thrust counterforce was withdrawn, the probe slowed down to achieve a "soft landing".
The far side of the moon is not as flat as the front of the moon, for the purpose of the target task, Chang'e-6 chose to land near the foot of the mountain, which greatly increases the difficulty of landing, not only to accurately locate in the air, but also to have superb obstacle avoidance ability, the terrain is dangerous, undulating, so that the soft landing faces the risk of "a misstep into a thousand hatreds". The same difficulty is that the direction of the earth is not directly visible during the moon landing, and all information transmission needs to be relayed by the relay satellite "Queqiao-2", and the whole process is almost "blind landing", which undoubtedly increases the risk.
In order to achieve a more stable flight of the probe, a more reasonable selection of the landing site, a more effective obstacle avoidance measures, and a more accurate landing point position in the process of power descent, the propulsion system needs to provide the main deceleration thrust for the probe, the control force for attitude control, and the translational thrust for hovering and obstacle avoidance. Because the aircraft is often in a weightless state in space, and the liquid propellant is in a floating state at this time, the 150N engine should first carry out ignition work to provide the lander with the initial propellant sinking force and support the main engine to start the ignition and decelerate, and as the landing/ascent combination continues to descend close to the lunar surface, the 150N engine adjusts its attitude, translates laterally, and accurately avoids obstacles to ensure the safe and stable "parking and storage" of Chang'e-6.
In order to meet the strict weight requirements of the detector, compared with the previous similar products, the overall weight of the engine is reduced by 42%, a single product is less than 1kg, which is lighter than two bottles of 500mL mineral water, and the aperture of the smallest position is thinner than the refill head, and at the same time, it can ensure tens of thousands of pulses and tens of thousands of seconds of steady-state working life, ensuring the high-precision, stable and reliable flight of Chang'e-6.
The strongest "guard" in a million
Each stage of the mission of the "Lunar Exploration Project" is a deepening of the previous stage of the mission and lays the foundation for the next stage of the mission. As a backup for Chang'e-5, how to ensure that the quality of Chang'e-6 is not affected during the long-term storage of Chang'e-6 is a new issue faced by the research and development team.
To this end, the R&D team has carried out a large number of analysis and experimental studies, discussed measures such as "storage and life extension research, replacement of overdue components, and replacement of new products", and put products into production in advance to prepare for new products. In order to maintain the consistency of the technical status, the research and development team also carried out long-term storage impact test analysis and life assessment of the product, carried out multiple rounds of special quality review, and the research and development team repeatedly scrutinized the flight procedures, plans and system parameters to ensure the long life and reliability of the product. Although there have been successful soft landings of Chang'e-3, Chang'e-4, Tianwen-5 and Tianwen-1, in order to smoothly land Chang'e-6 on the surface of the moon, it is necessary to select the most experienced and capable "escorts".
"Chang'e" set off to the moon again and again, and every time it arrived, what left us was amazement, and behind this was the intellectual breakthrough of the technical difficulties of liquid power and the cocoon of innovative achievements. These innovations have set up a ladder to the moon and allowed the Chinese to embark on their own path to lunar exploration.
Today, destined to be an unforgettable day, the Beijing Aerospace Flight Control Center, the last second was still immersed in a tense atmosphere, the moment of successful moon landing applause and cheers. However, for the people of the Sixth Academy, there was no moment of relaxation, the moon landed successfully, the life of the Moon Palace of "Chang'e" has just begun, and new challenges are ahead, so let's look forward to Chang'e-6 returning home with "soil".
Source: The Sixth Academy of China Aerospace Science and Technology Corporation